Various polymer films are used to make bags, pouches, packages and the like. A common practice is to join opposed polymer sheets together along a common line by heat sealing, wherein the sheets are pressed together between heated bars for a sufficient length of time and with sufficient pressure to cause fusion therebetween.
If a bag or package composed of a polymer film is heat sealed together on all sides, difficulties are frequently encountered in tearing the bag open. Many polymer films tend to stretch before tearing. Moreover, most films will not tear along a straight line, which presents a problem in opening of a bag or package.
A conventional method of forming polymer films is by blow extrusion, wherein the polymer is melted and forced through a circular die to produce a thin tube, which is inflated with a gas in the vicinity of the die and drawn away in a continuous process. Widely used polymers include low density polyethylene, usually modified with vinyl acetate, as well as ionomers, both of which provide a film having good heat sealing properties and excellent strength. Films composed entirely of either an ionomer or low density polyethylene, however, made by normal blow or other extrusion methods, cannot be torn along a straight line in the direction of extrusion. The film can be torn in such direction only with great difficulty and not in a straight line.